Researchers discover secret of weevil diamond-like coat

December 22, 2011 by Bob Yirka in Chemistry / Materials Science

Entimus imperialis. Image: René Limoges, Insectarium de Montréal

(PhysOrg.com) -- The diamond weevil (Entimus imperialis), also called sometimes as the Australian weevil, is a bug known throughout Australia as a pest, (another close relative resides in South America) as are most weevils in other parts of the world. What sets the diamond weevil apart though is its interesting appearance. Though they have a longer snout than most weevils, which allows them to embed their eggs deeply into plant chambers, that’s not what go them their name; instead, it’s the remarkable coat they seem to wear on their backs. It’s all black save for the rows of pits filled with what look like little diamonds. And not only do they look like diamonds, they act like them too, dazzling onlookers by reflecting light in a brilliant display of color that is eerily reminiscent of jewelry worn by us humans. But of course, they aren’t real diamonds, that would require pressuring carbon to a very high degree. Instead a team of European researchers have found, as they describe in their paper published in the Journal of the Royal Society Interface, that the diamonds on the weevil’s backs are in fact made of chitin, arranged in a diamond type arrangement.

Researchers and various other people have been puzzled for years as to how the diamond weevil manages to produce a coat that sparkles as well as any real diamond, but until recently, lacked the technology necessary to uncover the secret.

Now, using electron microscopy, this team discovered that the diamond-like material is actually made of nothing more than chitin, a long polymer derivative of glucose. Its most commonly found in anthropoids, mollusks and crustaceans as well as in a variety of insects. In this case, the diamond weevil.

Scanning electron microscopy of single scales of E. imperialis. (a) A single, intact scale. The upper side of the scale consists of a set of more or less parallel furrows (scale bar: 20 µm). (b) SEM image of a cross-section showing tilted sheets with hexagonal symmetry (scale bar: 2 µm). Image (c) J. R. Soc. Interface, doi:10.1098/​rsif.2011.0730

In most other animals, chitin appears as a dull whitish material, and is used by those higher up the evolutionary chain as an ingredient in medical and industrial products. In diamond weevils, it’s the way the material that is arranged that is different. For some as yet to be discovered reason, the gems in their coats are crystal structured in the same way as real diamonds, i.e. as photonic crystals.

Turns out, each little “gem” has crystal scales on it, each of which reflect a different wavelength of light at a different angle, producing the sparkling effect.

Now that the little bug’s secret has been revealed, other researchers will no doubt be looking into whether such gems might be made artificially and if so, if there might be any good use for them.

AbstractThe brilliant structural body colours of many animals are created by three-dimensional biological photonic crystals that act as wavelength-specific reflectors. Here, we report a study on the vividly coloured scales of the diamond weevil, Entimus imperialis. Electron microscopy identified the chitin and air assemblies inside the scales as domains of a single-network diamond (Fd3m) photonic crystal. We visualized the topology of the first Brillouin zone (FBZ) by imaging scatterometry, and we reconstructed the complete photonic band structure diagram (PBSD) of the chitinous photonic crystal from reflectance spectra. Comparison with calculated PBSDs indeed showed a perfect overlap. The unique method of non-invasive hemispherical imaging of the FBZ provides key insights for the investigation of photonic crystals in the visible wavelength range. The characterized extremely large biophotonic nanostructures of E. imperialis are structurally optimized for high reflectance and may thus be well suited for use as a template for producing novel photonic devices, e.g. through biomimicry or direct infiltration from dielectric material.